In the Norman L. Stauffer U.S. Pat. No. 4,185,191 issued Jan. 22, 1980, and assigned to the assignee of the present invention, a range determination system is shown which is usable in a through-the-lens camera for positioning the taking lens at a desired correct focus position with respect to a remote object within the field of view. The system utilizes a plurality of small detectors such as charge coupled devices (CCD) or Charge Injection Devices (CID) mounted in an array to receive radiation from the scene being viewed. The detectors are arranged in pairs with each pair being mounted behind a small lenslet so that each pair receives a view of the exit pupil of the taking lens but one of the detectors in each pair receives radiation primarily from a first portion of the taking lens while the other of the detectors in each pair receives radiation primarily from a different portion of the taking lens. The result is to create two similar curves indicative of the radiation distribution pattern from the scene being viewed. At the proper focus position, both of the detectors of each pair receive substantially the same amount of radiation and the two curves coincide, but as the object changes position with respect to the camera, the detectors in each pair receive different amounts of radiation and the two curves move with respect to each other to indicate an out-of-focus situation.
The accuracy of the system depends upon the matching of the detector pairs so that when they receive the same amount of radiation they produce substantially the same output signals. Unfortunately, it is extremely difficult to manufacture detector pairs that are always closely matched and accordingly in the past, attempts have been made to vary the output of one of the detectors in the pair so as to cause it to match the output of the other detector in the pair when both are subjected to the same radiation. For example, in the co-pending application of Norman L. Stauffer, Ser. No. 213,438, filed Dec. 5, 1980 now U.S. Patent No. 4,359,636 issued Nov. 16, 1982, a detector balance apparatus and method is shown in which portions of one or both of the detectors in a pair are shadowed by pins or strips in order to cause the outputs of the two detectors to be the same under the same conditions. While such apparatus as this is satisfactory to balance a detector pair, it is time consuming and expensive to perform this operation on all of the detector pairs particularly when, as often happens, a mismatch in detector pairs is caused by a manufacturing problem which mismatches all of the detectors in a certain batch.